# Pair-annihilation why does it occour? [duplicate]

Why does pair annihilation occur with particles and only their matching anti-particle? E.g., electrons and positrons, but not protons and positrons? What is the difference?

We (physicists) believe the reason is this: known symmetries and conservation laws. For example, the mutual annihilation of a proton and positron would remove $2\,e$ charge units from the Universe. This violates the conservation of charge principle, which can be seen to arise from the application of Noether's theorem to the global gauge symmetry of the classical electrodynamic Lagrangian, as discussed in QMechanic's answer to the question "Noether theorem and classical proof of electric charge conservation". Other similar conservation laws are conservation of color charges in QCD, conservation of lepton number, baryon number (quark number) and so forth. Your proposed reaction would also violate conservation of lepton number.